Hot-Rolled Flat Steel Product Produced from a Complex Phase Steel and Method for the Production Thereof

ABSTRACT

A hot-rolled flat steel product having a tensile strength of at least 1100 MPa, good elongation properties, and good deformation properties. The flat steel product is produced from a complex phase steel, which contains, in addition to iron and inevitable impurities (in % by weight), C: 0.13-0.2%, Mn: 1.8-2.5%, Si: 0.70-1.3%, Al: 0.01 to 0.1%, P: up to 0.1%, S: up to 0.01%, Cr: 0.25-0.70%, optionally Mo, the total of the Cr and Mo contents being 0.25-0.7%, Ti: 0.08-0.2%, B: 0.0005-0.005%, and a structure which consists of at the most 10% by volume of residual austenite, 10-60% by volume of martensite, at the most 30% by volume of ferrite and at least 10% by volume of bainite. Also, a method for producing such a flat steel product.

The invention relates to a hot-rolled flat steel product produced from a complex phase steel and a method for producing such a product.

EP 2 028 282 A1 discloses a cold-rolled flat steel product which is produced from a dual phase steel and which, in addition to a tensile strength of at least 950 MPa and good deformability, also has a surface quality which, using a simple production method, enables the flat product produced from this steel, in the uncoated state or a state provided with a covering which protects against corrosion, to be formed into a complex shaped component, such as a component of a vehicle bodywork. This is achieved in that the steel according to the invention comprises from 20 to 70% of martensite, up to 8% of residual austenite and as the remainder ferrite and/or bainite and contains (in % by weight): C: 0.10-0.20%, Si: 0.10-0.60%, Mn: 1.50-2.50%, Cr: 0.20%-0.80%, Ti: 0.02-0.08%, B: <0.0020%, Mo: <0.25%, Al: <0.10%, P: ≦0.2%, S: 0.01%, N: 0.012% with the remainder being iron and inevitable impurities. The flat steel products produced from such a steel in practice achieve tensile strengths of up to 1050 MPa.

Another possibility for producing a high-strength steel is described in EP 0 966 547 B1. According to the method explained therein, a steel which contains (in % by weight) 0.10-0.20% C, 0.30-0.60% Si, 1.50-2.00% Mn, max. 0.08% P, 0.30-0.80% Cr, up to 0.40% Mo, up to 0.20% Ti and/or Zr, up to 0.08% Nb, remainder Fe and inevitable impurities, is melted, cast to form slabs and subsequently rolled to form a hot-rolled strip. The rolling end temperature is above 800° C. The hot-rolled strip is subsequently cooled with a cooling speed over the runout rolling operation of at least 30° C./s, so that the conversion of the steel is carried out to the greatest possible extent in the bainite stage and a conversion of the steel into perlite is prevented. Proportions of martensite in the structure of the hot-rolled strip can further increase the tensile strengths. Furthermore, the comparatively rapid cooling contributes to the precipitation of extremely fine particles, by means of which the strength is further increased. The cooling operation is intended to be ended at a temperature below 600° C. by the strip being wound on a coiler and subsequently being further cooled in the coil. The hot-rolled strip obtained in this manner regularly reaches tensile strengths of up to 1150 MPa.

Against the background of the prior art explained above, an object of the invention was to provide a flat steel product in which further increased tensile strengths with good elongation properties and consequently inherently good deformation properties are combined. A method for producing such a flat steel product is also intended to be set out.

With reference to the steel, this object is achieved with a complex phase steel having the composition and grain structure set out in claim 1.

The method which achieves the object set out above according to the invention is characterised by the measures set out in claim 14.

Advantageous embodiments of the invention are set out in the dependent claims and are explained in detail below as well as the general idea of the invention.

The complex phase steel used for the production of a hot-rolled flat steel product according to the invention contains in addition to iron and inevitable impurities (in % by weight), C: 0.13-0.2%, Mn: 1.8-2.5%, Si: 0.70-1.3%, Al: up to 0.1%, P: up to 0.1%, S: up to 0.01%, Cr: 0.25% -0.70%, optionally Mo, the total of the Cr and Mo contents being 0.25-0.7%, Ti: 0.08-0.2% and B: 0.0005-0.005%.

Owing to the complex phase structure thereof, a flat steel product hot-rolled from the steel according to the invention has a high strength, with good elasticity at the same time. The structure of a flat steel product according to the invention, owing to the alloy thereof selected within narrow limits, is characterised in that the structure thereof comprises at the most 10% by volume of residual austenite, 10-60% by volume of martensite, at the most 30% by volume of ferrite and bainite as the remainder, the proportion being intended to be at least 10% by volume. Perlite is in any case present in a flat steel product according to the invention in ineffective traces, the perlite proportion being minimised where possible.

Flat steel products according to the invention thus reach in the hot-rolled state a tensile strength Rm, which is greater than 1100 MPa, in particular regularly at least 1150 MPa and more, and a yield point Re of also regularly at least 720 MPa. For its elongation at break A80, it is at the same time possible to ensure values of more than 7%, in particular more than 8%. This high strength, paired with the comparatively good elongation properties, have been achieved by means of the adjustment of the complex phase structure according to the invention.

Carbon is added to the complex phase steel used according to the invention in order to harden the structure and to form extremely fine precipitations. Owing to the presence of C in the contents predetermined according to the invention of 0.13% to 0.2% by weight, a martensite and bainite proportion which is sufficiently high for the desired hardness is thus achieved in the structure. At contents of more than 0.20% by weight, carbon prevents the occurrence of the desirably high bainitic structure proportion. Relatively high C contents also have a negative effect on the weldability, which is particularly significant for the use of the material according to the invention, for example, in the field of automotive construction. The advantageous effect of carbon in a steel which is used to produce a flat steel product according to the invention can be used in a particularly reliable manner when the C content is 0.15-0.18% by weight, in particular a maximum of 0.17% by weight.

Manganese at a content of at least 1.8% by weight delays the transformation and brings about the formation of hard, strength-increasing transformation products. The occurrence of martensite is thus promoted by the presence of Mn. In order to prevent inadmissibly high micro-segregations, the content is limited according to the invention to a maximum of 2.5% by weight, the advantageous influences of Mn then appearing in a particularly reliable manner when the Mn content of the steel according to the invention is limited to 2.05 to 2.2% by weight.

In a steel which is used according to the invention, Si also serves to increase the strength, on the one hand by promoting the solid solution hardening of the ferrite or bainite and, on the other hand, by stabilising the residual austenite. The residual austenite proportion contributes to increasing the elasticity and strength (TRIP effect). In order to achieve the desired high mechanical characteristic values, steel according to the invention has 0.70-1.3% by weight of Si, in particular at least 0.75% by weight of Si. The strength and elasticity-increasing effect occurs in particular when the Si content of a steel according to the invention is at least 0.75% by weight, in particular at least 0.85% by weight. With regard to the fact that a flat product produced from a steel according to the invention is intended to have an optimal surface quality for further processing and any coatings which may be applied if necessary, the upper limit of the Si content has been determined to 1.3% by weight at the same time. When these upper limits are complied with, the danger of grain boundary oxidation is also minimised. An unfavourable influence of Si on the properties of the steel used according to the invention can be prevented with even greater reliability by the Si content of the steel according to the invention being limited to 1.1% by weight, in particular 0.95% by weight.

The steel which the flat steel product according to the invention consists of, is Al-stabilised. Aluminium is used in the melting of a steel according to the invention for deoxidation and for binding nitrogen which may be contained in the steel. To this end, Al in contents of less than 0.1% by weight may be added to the steel according to the invention, if necessary, the desired effect of Al then occurring in a particularly reliable manner when the contents thereof are in the range from 0.01-0.06% by weight, in particular 0.020-0.050% by weight.

Phosphorus can be used to further increase the solid solution hardening, but should not exceed a content of 0.1% by weight for reasons of weldability, owing to the otherwise increasing risk of the formation of segregations.

With contents of sulphur which are below the upper limit predetermined according to the invention, the formation of MnS or (Mn, Fe)S in the steel used according to the invention is suppressed so that good elasticity of the flat steel product according to the invention is ensured. This is particularly the case when the S-content is below 0.003% by weight.

At contents of at least 0.25% by weight, chromium prevents the formation of ferrite and perlite. Accordingly, it promotes the formation of a hardening structure and consequently the strength of the steel used for the flat steel product according to the invention. In order not to excessively delay the transformation, the content thereof should be limited to a maximum of 0.7% by weight. By the Or content of a steel according to the invention being limited to 0.7% by weight, the risk of the occurrence of grain boundary oxidation is reduced and the good elongation properties of the steel according to the invention are ensured. When this upper limit is complied with, a surface of the flat steel product produced from the steel is also achieved which can readily be provided with a metallic coating.

The optionally provided contents of molybdenum contribute in the same manner as Cr to increasing the strength of a steel according to the invention, by promoting the formation of extremely fine precipitations and martensite in the structure of the steel. The presence of Mo does not have a negative effect on the ability to coat the flat product with a metal coating and the elasticity thereof. Practical tests have shown that the positive effects of Mo can be used in a particularly effective manner up to contents of 0.25% by weight, in particular 0.22% by weight, including from a cost point of view. Thus, Mo contents of 0.05% by weight already have a positive effect on the properties of the steel according to the invention.

However, in order to prevent the elongation at break being influenced in a negative manner by an excessively high martensite proportion, the total of the Cr and Mo contents in a steel used according to the invention is limited to from 0.25% to 0.7% by weight.

With titanium in contents of from at least 0.08% to a maximum of 0.2% by weight, in particular 0.09% -0.15% by weight, in steel according to the invention the formation of extremely fine precipitations in the form of TiC or Ti(C, N) with a hardening effect can be promoted and grain refinement can be brought about. Another positive effect of Ti involves the binding of any nitrogen which may be present so that the formation of boron nitrides in the steel according to the invention is prevented. Owing to the presence of Ti consequently, in the case of an addition of boron to increase strength, it is also ensured that the boron can fully develop its effect in the dissolved state. The positive effect of Ti can be used in a particularly reliable manner in a steel according to the invention when the Ti content thereof is 0.11-0.13% by weight.

In steel used according to the invention, boron improves the hardenability when B is present in contents of from 0.0005-0.005% by weight. In austenite, boron segregates at the grain boundaries and prevents the formation of ferrite and perlite. In this instance, boron brings about a significant increase of the strength with only a slight reduction of the deformability. The favourable influences of B on the alloy according to the invention are produced in a particularly reliable manner when the B content of the steel according to the invention is determined to 0.001% -0.002% by weight.

Flat steel products which are produced in a manner according to the invention are distinguished by a particularly high level of grain fineness, a high yield point and increased strength. The proportions of martensite, bainite and extremely fine precipitations contained in the structure thereof contribute to the high degree of strength. The residual austenite and ferrite portions of the structure ensure the good elongation properties thereof.

If flat steel products produced according to the invention are intended to be protected particularly against corrosion, the hot-rolled strips may be provided with a metallic protective coating before or after they are shaped to form a component. This can be carried out by means of hot-dip galvanising, or electrolytic coating.

During the production according to the invention of a hot-rolled flat steel product according to the invention having a tensile strength of more than 1100 MPa and the above explained structure, a steel melt having a composition which falls under the alloy of the steel used according to the invention is first cast to form a preliminary product which is typically a strand which is cut into slabs or thin slabs.

Subsequently, the preliminary product is heated to a temperature of 1150-1350° C. in order to ensure for the subsequent hot rolling a completely austenitic structure of the steel and to bring the precipitations into a solution.

Based on this heating temperature, the preliminary product is then hot-rolled to form a hot-rolled strip, the end temperature of the hot rolling being 800-950° C. The rolling end temperature should be in the range of the homogeneous austenite and consequently not be below 800° C. in order to keep deformation-induced precipitations low and to enable the development of the desired structure composition.

After the hot rolling, the hot-rolled strip which is obtained is cooled to the coiling temperature selected in each case at a cooling speed of at least 30° C./sec. The cooling conditions are intended to be selected in such a manner that a transformation to perlite is prevented and the transformation is carried out to the greatest possible extent in such a manner that the high bainite proportions and the proportions predetermined according to the invention of martensite and residual austenite are obtained.

The cooling operation is ended when the range of the coiling temperature of 400-570° C. predetermined according to the invention is achieved, in which the bainite stage of the steel according to the invention is achieved. The hot-rolled strip which is cooled accordingly is then wound to form a coil and is further cooled in the coil. Further transformations into bainite and martensite and the formation of precipitations occur.

Owing to its particular combination of high strength and good expansion properties, steel according to the invention is particularly suitable for producing profile-members which are highly loaded in practical use and for crash- and strength-relevant components for vehicle bodyworks.

EMBODIMENTS Test 1

Under laboratory conditions, a steel having the composition set out in Table 1 was melted and cast into blocks.

The blocks were subsequently heated to 1270° C. and, starting from this temperature, hot-rolled to form hot-rolled strip having a thickness of 2.5 mm. The hot rolling end temperature was 900° C.

The hot-rolled strip obtained was slowly cooled in an oven after the hot rolling operation at a cooling speed of 80° C./sec. and at a temperature of 490° C. in order to simulate the cooling in the coil.

The hot-rolled strip obtained had transversely relative to the rolling direction a tensile strength Rm of 1192 MPa and an elongation A80 of 10.5%. The structure obtained comprises 35-40% by volume of martensite, approximately 5% by volume of ferrite, 6% by volume of residual austenite and the balance comprises bainite.

For a first comparison, the hot-rolled strips produced in the manner explained above, after the hot rolling, were first cooled to a temperature of 75° C. and subsequently slowly further cooled to ambient temperature in the oven in order to also simulate the cooling in the coil in this instance. The hot-rolled strips obtained in this manner had a tensile strength Rm of 1550 MPa and a comparatively low elongation A80 of 5.9%. They were primarily martensitic.

For a second comparison, the above-explained hot-rolled strips, after the hot rolling operation, were first cooled to a temperature of 600° C. corresponding to the “coiling temperature” and subsequently slowly cooled again to ambient temperature in order to simulate the cooling in the coil. The hot-rolled strips obtained in this manner had a tensile strength Rm of 955 MPa and an expansion A80 of 15.5%. The structure consisted of ferrite having a perlite proportion of 25-30% by volume.

Test 2

Also under laboratory conditions, a steel having the composition set out in Table 2 was melted and cast into blocks. In contrast to the steel examined in the first test, this steel additionally contained 0.25% by weight of Mo.

The blocks were subsequently heated to 1270° C. and, starting from this temperature, were hot-rolled to form hot-rolled strip having a thickness of 2.5 mm. The hot rolling end temperature was 900° C.

The hot-rolled strip obtained, after hot rolling, was cooled to a “coiling temperature” of 550° C. at a cooling speed of 80° C./sec., from which the coil cooling has again been simulated in the manner already described above.

The hot-rolled strip obtained had a tensile strength Rm of 1180 MPa and an elongation A80 of 11%. The structure thereof had a martensite proportion of 35-40% by volume, a residual austenite content of 7.5% by volume, a ferrite content of 10% by volume and, as the remainder, bainite.

Test 3

For an operational test 3a to 3c, a steel having the alloy set out in Table 3 according to the invention was melted and cast to form a strand. The slabs separated from the strand were subsequently reheated to a temperature of approximately 1260° C., subsequently hot-rolled at a hot rolling end temperature WET to form hot-rolled strips having a thickness D and finally cooled at a cooling rate V_(T) to a coiling temperature HT, at which they were wound to form a coil. The parameters adjusted in each case and the mechanical properties of the hot-rolled strips obtained (determined transversely relative to the roller direction) are set out in Table 4.

It has been found that the hot-rolled strip obtained during the operational test 3c, owing to the excessive coiling temperature as a result of a high ferrite proportion (and perlite), had a significantly lower tensile strength than the hot-rolled strips obtained in the tests 3a and 3b and wound in the temperature range according to the invention.

Test 4

For an additional operational test V carried out for the purposes of comparison, a steel having the alloy which is set out in Table 5 and which is not in accordance with the invention owing to the clearly excessively low Si content and its also excessively low contents of Mn, Cr and Ti was melted and cast to form a strand from which slabs have been separated. The slabs were subsequently reheated to a temperature of 1250° C., subsequently hot-rolled at a hot rolling end temperature WET to form a hot-rolled strip having a thickness D and finally cooled at a cooling rate V_(T) to a coiling temperature HT at which it was wound to form a coil. The parameters adjusted in each case and the mechanical properties of the hot-rolled strip obtained are set out in Table 6.

It has been found that, although the hot-rolled strip obtained in the comparison test 5 had a high tensile strength, the elongation properties thereof were inadequate.

TABLE 1 C Si Mn P S Al Cr Mo N Ti B 0.15 0.89 2.06 0.013 0.004 0.041 0.36 — 0.0047 0.12 0.0011 Remainder iron and inevitable impurities, values in % by weight

TABLE 2 C Si Mn P S Al Cr Mo N Ti B 0.155 0.87 1.94 0.013 0.006 0.029 0.36 0.25 0.0050 0.11 0.0009 Remainder iron and inevitable impurities, values in % by weight

TABLE 3 C Si Mn Al Cr Mo N Ti B 0.16 0.86 2.05 0.033 0.33 — 0.004 0.12 0.002 Remainder iron and inevitable impurities, values in % by weight

TABLE 4 In accordance WET D V_(T) HT Re Rm A80 with Test [° C.] [mm] [° C./s] [° C.] [MPa] [MPa] [%] invention? 3a 900 3 50 470 794 1194 11.0 YES 3b 900 1.6 90 500 825 1203 9.6 YES 3c 910 1.6 85 590 693 789 14.7 NO

TABLE 5 C Si Mn Al Cr Mo N Ti B 0.12 0.08 1.40 0.029 0.23 — 0.004 0.04 0.001 Remainder iron and inevitable impurities, values in % by weight

TABLE 6 In accordance WET D V_(T) HT Re Rm A80 with Test [° C.] [mm] [° C./s] [° C.] [MPa] [MPa] [%] invention? V 860 1.6 125 50 1012 1284 6.3 NO 

1. A hot-rolled flat steel product having a tensile strength of at least 1100 MPa and good elasticity and produced from a complex phase steel, comprising in addition to iron and inevitable impurities (in % by weight), C: 0.13-0.2%, Mn: 1.8-2.5%, Si: 0.70-1.3%, Al: 0.01to 0.1%, P: up to 0.1%, S: up to 0.01%, Cr: 0.25-0.70%, optionally Mo, the total of the Cr and Mo contents being 25-0.7%, Ti: 0.08-0.2% B: 0.0005-0.005%, and having a structure comprising at the most 10% by volume of residual austenite, 10-60% by volume of martensite, at the most 30% by volume of ferrite and at least 10% by volume of bainite.
 2. The flat steel product according to claim 1, wherein the C content of the complex phase steel is from 0.15 to 0.18% by weight.
 3. The flat steel product according to claim 1, wherein the C content of the complex phase steel is at the most 0.17% by weight.
 4. The flat steel product according to claim 1, wherein the Mn content of the complex phase steel is 2.05-2.2% by weight.
 5. The flat steel product according to claim 1, wherein the Si content of the complex phase steel is at least 0.75% by weight.
 6. The flat steel product according to claim 1, wherein the Si content of the complex phase steel is at the most 1.1% by weight.
 7. The flat steel product according to claim 5, wherein the Si content of the complex phase steel is at least 0.85% by weight.
 8. The flat steel product according to claim 1, wherein the Si content of the complex phase steel is at the most 0.95% by weight.
 9. The flat steel product according to claim 1, wherein the Al content of the complex phase steel is from 0.02 to 0.05% by weight.
 10. The flat steel product according to claim 1, wherein, the Cr content of the complex phase steel is from 0.30 to 0.40% by weight.
 11. The flat steel product according to claim 1, wherein the Ti content of the complex phase steel is from 0.09 to 0.15% by weight.
 12. The flat steel product according to claim 9, wherein the Ti content of the complex phase steel is from 0.11 to 0.13% by weight.
 13. The flat steel product according to claim 1, wherein the B content of the complex phase steel is 0.001-0.002% by weight.
 14. A method for producing a hot-rolled flat steel product, comprising the following operating steps: casting a complex phase steel produced according to claim 1 to form a preliminary product, heating the preliminary product to a temperature of 1150-1350° C., hot rolling the preliminary product to form a hot-rolled strip, the end temperature of the hot rolling being 800-950° C., cooling the hot-rolled strip obtained at a cooling speed which is at least 30° C./s, and coiling the hot-rolled strip obtained at a coiling temperature of 400-570 ° C.
 15. The flat steel product according to claim 10, wherein the Ti content of the complex phase steel is from 0.11 to 0.13% by weight.
 16. The flat steel product according to claim 6, wherein the Si content of the complex phase steel is at least 0.85% by weight. 